Definition The exendins are peptides that are found in the salivary secretions of the Gila monster and the Mexican Bearded Lizard, reptiles that are endogenous to Arizona and Northern Mexico. Exendin-3 is present in the salivary secretions of Heloderma horridum (Mexican Beaded Lizard), and exendin-4 is present in the salivary secretions of Heloderm suspectum (Gila monster) 1.
Related Peptides
The GLP-1 structurally related peptides exendin-4 and exendin (9-39) amide were found to act, in rat liver and skeletal muscle, as agonist and antagonist, respectively, of the GLP-1 (7-36) amide effects on glucose metabolism 2.
Discovery
In 1982, it was observed that the crude venom of the Gila monster Heloderma suspectum was a potent pancreatic secretagogue. Purification and sequencing of the active factors mediating this effect led to the discovery of the peptides helodermin and exendin-4 3.
Structural Characteristics
The exendins have some sequence similarity to several members of the glucagon-like peptide family, with the highest homology, 53%, being to GLP-1[7-36] NH2 2. An amino acid sequencing assay for peptides containing an amino-terminal histidine residue (His1) was used to isolate a 39-amino acid peptide, exendin-4, from H. suspectum venom. Exendin-4 differs from exendin-3 by two amino acid substitutions, Gly2-Glu3 in place of Ser2-Asp3, but is otherwise identical. The structural differences make exendin-4 distinct from exendin-3 in its bioactivity 4.
Mode of Action
In normal rats, exendin-4, like GLP-1 and insulin, enhanced glucose uptake. This effect, which is mediated to a certain extent by some kinases (PI3K/ PKB, p70s6k and MAPKs), may be caused by the peptide acting, at least in part, through the muscle GLP-1 receptors. Exendin-9 also stimulated the same kinases, except for PKB, but failed to modify basal glucose uptake 5. Pharmacological studies have led to reports that exendin-4 can act at GLP-1 receptors in vitro on certain insulin-secreting cells, at dispersed acinar cells from guinea pig pancreas, and at parietal cells from stomach; the peptide is also reported to stimulate somatostatin release and inhibit gastrin release in isolated stomach.
Exendin-3 and exendin-4 were reportedly found to stimulate cAMP production in, and amylase release from, pancreatic acinar cells.1
Functions
Like GLP-1 (7-36) amide, exendin-4 increased glycogen synthase activity and glucose incorporation into glycogen in both tissues and also stimulated exogenous D -glucose utilization and oxidation in muscle. These effects of GLP-1(7-36) amide and exendin-4 were inhibited by exendin (9-39) amide 2. Novel modified exendins and exendin agonists having an exendin or exendin agonist linked to one or more polyethylene glycol polymers, and related products and methods are useful, for example, in the treatment of diabetes, including Type 1, Type 2, and gestational diabetes, in the treatment of disorders which would be benefited by agents which modulate plasma glucose levels or suppress glucagon secretion, and in the treatment of disorders which would be benefited by the administration of agents useful in modulating the rate of gastric emptying or food intake, including obesity, eating disorders, insulin-resistance syndrome, and triglyceride levels, and to treat subjects suffering from dyslipidemia. The methods are also useful for lowering plasma lipid levels, reducing cardiac risk, reducing appetite, and reducing the weight of subjects.1
References
1. Eng J, Andrews PC, Kleinman WA, Singh L, Raufman JP (1990). Purification and structure of exendin-3, a new pancreatic secretagogue isolated from Heloderma horridum venom. J Biol Chem., 265(33):20259-20262
2. Alcántara AI, Morales M, Delgado E, López-Delgado MI, Clemente F, Luque MA, Malaisse WJ, Valverde I, Villanueva-Peñacarrillo ML (1997). Exendin-4 Agonist and Exendin(9-39)amide Antagonist of the GLP-1(7-36)amide Effects in Liver and Muscle. Archives of Biochemistry and Biophysics., 341(1):1-7.
3. Pohl M, Wank SA (1998). Molecular cloning of the helodermin and exendin-4 cDNAs in the lizard. Relationship to vasoactive intestinal polypeptide/pituitary adenylate cyclase activating polypeptide and glucagon-like peptide 1 and evidence against the existence of mammalian homologues. J Biol Chem., 273(16):9778-9784.
4. Eng J, Kleinman WA, Singh L, Singh G, Raufman JP (1992). Isolation and characterization of exendin-4, an exendin-3 analogue, from Heloderma suspectum venom. Further evidence for an exendin receptor on dispersed acini from guinea pig pancreas. J Biol Chem., 267(11):7402-7405.
5. Sancho V, Trigo MV, González N, Valverde I, Malaisse WJ, Villanueva-Peñacarrillo ML (2005). Effects of glucagon-like peptide-1 and exendins on kinase activity, glucose transport and lipid metabolism in adipocytes from normal and type-2 diabetic rats. J Mol Endocrinol., 35(1):27-38.